Optical instruments such as tachymeters or total stations normally comprise a lens or optical device such as a telescope and an image sensor, as well as actuators to align the optical instrument with a target. When the optical instrument is aligned with the target, angle measurements and distance measurements can be taken to allow the position of the target to be determined.
When aligning the optical instrument with the target, a visual control of the alignment is normally carried out using an eyepiece having crosshairs or using a display on which an image of the target captured by the image sensor and crosshairs can be seen.
Such optical instruments can be used not only to determine the position of a non-moving object; they are also suitable for tracking a moving target. When tracking a moving target the optical instrument is aligned with a target, for example. The optical instrument comprises an image analysis device to identify the target on a sequence of images of the image sensor and also comprises servomotors which allow the optical instrument to always be realigned with the moving target in order to track it.
Areas of application include, for example, road construction, in which the position of a bulldozer, for example, has to be determined and control commands for accurate control can be issued to the bulldozer on the basis of the determined position. Here, it is often the case that a cooperative target is connected to the object, such as a reflector, with which the optical instrument is aligned using a laser for distance measurement. Reflected laser light is detected and used for tracking the target. Moreover, the tracking can also be performed with light in the visible spectral range.
In practical applications, however, it is often a problem that at close range the image of the target moves at a high angular speed on the image sensor, and the servomotors of the optical instrument or the imaging region on the image sensor by adjusting the lens are not sufficient for continuously tracking the target. Furthermore, covering may occur when the target moves closer to objects located near the optical instrument, such as trees, vehicles, walls etc. This problem is made worse by the fact that for accurate angle measurements when tracking the target a lens is often used which covers a narrow angle range, and therefore finding the object by way of a visual control or by an automatic tracking of the object is difficult.
In order to solve this problem, a wide angle lens can be provided parallel to the optical device for tracking, by means of which the target is easier to find and can then be targeted with the tracking lens. This, however, is cumbersome since different lenses have to be provided for different purposes. The parallel offset is disadvantageous in terms of measuring. For accurate measurements, an extremely complex calibration would in this case be necessary.